After 50 do we get Merit Points like in FFXI or are we just dinging 50 and that's it?

Nothing at this time. Once you hit level 50, all XP gain stops. You'll be 0/562000 until otherwise noted by the development team, i.e. level cap increases or something else. So definitely keep this in mind when leveling, since you don't want quest XP from the main story to go to waste.

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Also, I've heard that once you open a job (WHM, BLM, SCH) whatever it is, you start leveling it at 30. Now, if you WAIT to open that job until 35 or 40, do you get throttled down back to 30?

Job XP is tied in to the XP of the class it is a part of. So even if you, for whatever reason, decided to save unlocking a job until later, it would still be at the same level as its corresponding class.